Fiber reinforced plastic pipe has come into reasonably extensive use in recent years for handling corrosive materials, petrochemicals and the like, where metallic pipe is unsuitable. Glass fiber reinforcements are employed so that pipe can withstand appreciable pressures. Epoxy resins, commonly with a thin liner resistant to chemicals, are often used. The liner may be an unreinforced epoxy resin somewhat flexibilized for a good sealing. The pipes are formed by winding rovings of glass fiber coated with epoxy resin in helical paths around a cylindrical mandrel and curing the resin. Such pipes can be made economically and it is also desirable to make economical high pressure fittings for such pipes, such as tees and elbows.
Techniques have been developed for economically winding pipe elbows which are essentially sharply curved sections of pipe having two ends. Economical techniques are desirable for winding pipe tees since they have a much more complicated geometry Unlike an elbow with two ends, a pipe tee has three ends. This greatly complicates the winding problems since it is important to cover all areas of the tee with a sufficient thickness of fibers with proper orientation for resisting the complex stress distributions in a tee without excess thickness being built up in other areas.
The patterns used for winding the tee must keep fibers in contact with or very close to the underlying mandrel liner or fibers to prevent voids or "bridging" of fibers across concave regions. Such voids or bridging can result in low strength in the bridged areas and require excessive quantities of reinforcement for resisting operating pressures.
Service requirements for fiber reinforced plastic pipe have increased over the years and it is desirable to have pipe and fittings with pressure ratings of as much as 2,000 psi (140 kg/cm.sup.2). To provide factors of safety, fittings are designed with short-term burst strength of 4,000 psi (280 kg/cm.sup.2) or more. Such requirements force substantial wall thicknesses for fittings, and winding of excess materials in areas where it is not needed may significantly increase the cost of such a fitting.
Many commercially available tees are made by hand by laying up a montage of strips of woven fabric. Such assembly techniques are costly since the woven glass fabric is expensive and a large amount of hand labor is required. Quality control of such assembly procedures is also difficult and costly. It is preferable to employ relatively inexpensive glass rovings (bundles of individual fibers) and short glass fibers properly oriented for optimum strength rather than costly woven fabric. Such a technique must cover all areas of the tee with an adequate thickness of material with proper orientation to resist the complex stress distribution in a pipe tee without excessive waste or thickness in some less critical areas of the tee.
One area of particular concern because of adverse stress distributions is known as the triangle, diamond or diaphragm area. It comprises a roughly triangular area on each side of the tee near where the three arms of the tee intersect. This area tends to be relatively flat and subject to biaxial stresses which are large and hard to resist in a non-ductile material such as fiber reinforced plastic.
Techniques for winding pipe tees are described in U.S. Pat. Nos. 4,506,918, 4,601,496, and 2,601,770. Techniques provided therein include winding of rovings of glass fibers in a variety of patterns that assure total 1 coverage of the triangular diaphragm as well as other portions of the tee body. U.S. Pat. No. 4,601,496 describes use of a special tape having several synthetic warp strands, such as nylon, serving as a carrier web for short parallel bundles of straight glass fibers as a weft. The carrier fibers have very little strength as compared with the glass and hence are not reinforcing fibers in the completed tee.
A typical tape suitable for winding a pipe tee has about a dozen warp strands of nylon spaced apart across the width of the tape for supporting the weft. The weft is formed of bundles of parallel glass fibers about five to six centimeters long. The bundles are made of short glass fibers ending at the end of the bundle, as distinguished from a woven fabric in which the weft strands are continuous fibers repeatedly doubled back on themselves at a selvage. The tape is also distinguished from fabric in that its strength is unidirectional along the length of the weft fibers, whereas woven fabric has biaxial strength. The warp strands of the tape are knitted into a series of interlocking loops with each loop loosely holding a bundle of glass fibers. A typical unidirectional tape has sufficient glass fibers to weigh about 430 grams per meter.
Such a tape is particularly useful for forming a pipe tee since the loosely knitted synthetic warp strands can stretch and slip laterally for covering the tape without bunching or deforming the bundles of glass fibers. The loose looping also permits skewing, that is, when the tape is wound on a mandrel the bundles of glass fibers need not remain perpendicular to the warp strands, although the bundles of glass fibers remain essentially parallel to each other. This is significant so that the tape can lie against the curved surfaces of the tee and the bundles of glass fibers can be maintained in alignments that are parallel to the principal direction of stress, thereby forming a strong pipe tee with a minimal use of material.
Typically such a tape is interleaved with a band of rovings of continuous glass fibers. A band of fibers about five to six centimeters wide impregnated with epoxy resin is wound onto the tee along with a layer of such tape. The tape can be dry, with resin supplied in the finished product by the impregnated band of glass rovings, or may also be coated with epoxy resin.
The tees wound in accordance with the aforementioned patents were formed on a mandrel with "lugs" at each end of the run and branch so that the windings of rovings and tape went off the end of the tee and were wrapped around the lug for achieving a desired angle of winding across the tee. When one winds a pipe tee for 2,000 psi (280 kg/cm.sup.2) service, wall thicknesses become quite high. For example, a nominal six inch (15 cm.) pipe tee may have a wall thickness of six to seven centimeters and winding excess material on the lugs can be difficult because of the large thickness, and furthermore the trimming of all of the excess material from the ends of the tee may be prohibitedly costly. It is therefore preferable to have the tee wound on a mandrel where the windings do not extend appreciably beyond the ends of the tee and little, if any, trimming is required for minimizing waste of material.
Suitable windings can easily be made in major portions of the tee such as, for example, at the ends of the run and branch where circumferential windings of the rovings and tape provide appropriate strength. As usual, difficulties are encountered in providing adequate strength and appropriate directions in the triangle area or diaphragm where the run and branch meet.
In practice of this invention, a row of spikes is placed in the diaphragm area and the rovings and tape are wound through the row so as to change direction in the spaces between spikes, thereby assuring appropriate orientation of reinforcing fibers in this region without excessive buildup of material in other areas.
A type of "spikes" which are probably better characterized as "bumps" have been previously employed in the triangle area of a fiber reinforced plastic pipe tee. These bumps, which are illustrated in FIGS. 1 and 2, were short cones with a base diameter of 8 mm and a height of 7.6 mm. The cones were spaced apart 9.5 mm in each of two rows which were about 22.2 mm apart center to center. The cones were integral with a 5 cm wide flat base about 1.5 to 2 mm thick.
Such a tee was made by forming a principal structural layer comprising patches of fabric and windings of bidirectional woven roving. Pieces of fabric were cut to appropriate shapes for covering various areas on the tee, wetted with resin, and laid in place by hand. Next layers of woven roving three or four inches wide and wetted with resin were spirally wrapped around the mandrel to form the principal structural layer of the tee. This was "tied down" with a nylon veil.
After the windings and patches were in place to form the structural body of the tee, a pad with two rows of "spikes", as described above and illustrated in FIGS. 1 and 2, was placed in each of the triangle areas on opposite sides of the tee. The two rows of "spikes" on the pads extended parallel to the run axis. The pads were tied down with windings of glass roving under substantial tension.
Windings of rovings alone were then made around the structural layer with some windings being made from around the back of the run to the row of "spikes" where the direction of winding was reversed and the rovings returned around the back of the run. A generally V-shaped pattern of windings was made, extending as far toward the branch as possible. This was followed with windings of rovings alone in patterns similar to those used to build up the structural layer of the tee.
The purpose of the overwrap of rovings onto the pipe tee was primarily cosmetic. The patches and woven rovings in the underlying structural body of the tee appear irregular, porous and undesirable to customers. The overwinding of rovings hides the tape and fabric, making a more attractive and saleable product. Such winding of rovings does contribute to the strength of the tee, but the principal structural strength was provided by the body of interleaved tape and rovings and the fabric patches underlying the bumps.
Such tees were made in sizes ranging from 6 to 16 inches nominal (15 to 40 cm) with a pressure rating of 150 psi (10.5 kg/cm.sup.2). These strips with two row of bumps ranged from 11.4 cm long with 11 bumps in each row for a 6 inch nominal pipe tee to a strip 18 cm long with 29 bumps in each row for a 16 inch nominal tee. Such an arrangement is totally unsuitable for high pressure tees as provided in practice of this invention.
For purposes of description in this specification, it is convenient to adopt a nomenclature representing various portions of a pipe tee. The following glossary of terms is therefore adopted:
run: the straight portion of the tee through which fluid can flow in a straight path; PA1 branch: the cross member of the tee transverse to the run through which fluid can flow in a right angle path between the branch and run; PA1 tee diameter: the nominal diameter of the pipe with which the tee is used. It is also the nominal diameter of the run and branch PA1 mid-plane: the plane of symmetry through the tee including the axes of the run and branch; PA1 side: a portion of the tee on one side of the mid-plane; PA1 back: a portion extending along the run of the tee on the opposite side of the run from the branch; PA1 front: a portion of the run facing in the same direction as the branch; PA1 crotch: each of the two portions where direction changes between the branch and an end of the run of the tee; PA1 diaphragm: a generally triangular area on each side of the body of the tee adjacent the intersection of the run and branch and more or less parallel to the mid-plane. This may also be referred to as the triangle or diamond.